1. Field of the Invention
This invention relates to a mobile wireless terminal connected to a base station via wireless, and thereby capable of being connected to a communication network. More particularly, this invention relates to a mobile wireless terminal featured by a handover method of switching connection from one base station to another, a program used for a handover, and a handover method.
2. Description of the Related Art
A mobile terminal communication system of a cellular type requires handover processing (handoff processing) of changing from one base station to another without interruption of communications.
There have been proposed various methods as the handover processing. In each method, a mobile wireless terminal generally selects, with reference to a received electric field strength, a base station having a higher received electric field strength and performs communications with the selected base station. In this event, the mobile wireless terminal stores and holds therein an activation condition for the handover processing as a threshold value, and executes the handover processing when the received electric field strength decreases below the threshold value.
FIG. 1 is an explanatory diagram illustrating handover execution processing by means of a handover execution threshold value.
As illustrated in FIG. 1, the handover processing is executed by the mobile wireless terminal depending on a height of the handover execution threshold value. When the handover processing is executed at a high communication level (namely, a high threshold level), it is possible to increase resistance against noise. In this case, execution times of handover processing should be increased, as shown in FIG. 1.
On the other hand, when the handover processing is executed at a low communication level (namely, a low threshold level), the resistance against the noise becomes weak. In this case, execution times of the handover processing can be decreased.
The handover execution threshold value is usually determined on designing the mobile wireless terminal in consideration of services and standards to be applied.
As examples of the technology relating to the above-mentioned mobile terminal communication system of the cellular type, there are proposed technologies described in Japanese Unexamined Patent Application Publication (JP-A) No. 2001-112039 and Japanese Unexamined Patent Application Publication (JP-A) No. 2007-251652.
Specifically, Japanese Unexamined Patent Application Publication (JP-A) No. 2001-112039 describes a mobile wireless terminal which is selectively connected in time division multiplexing fashion via wireless to a plurality of base stations. In this event, each of the base stations can be connected to a network. Specifically, the mobile wireless terminal includes: electric field strength detecting means for detecting a received electric field strength of a radio signal transmitted from each of the plurality of base stations; and base station detecting means which is connected to the electric field strength detecting means to detect one of the base stations with reference to the received electric field strength of the each of the plurality of base stations. The base station detecting means selects, as the one of the base stations, a base station which tends to increase in a received electric field strength. In addition, handover control means is included in the mobile wireless terminal to preferentially wirelessly connect the base station detected by the base station detecting means, when a communication state of communications with a connected base station is degraded in comparison with a condition set in advance, instead of the connected base station.
Japanese Unexamined Patent Application Publication (JP-A) No. 2007-251652 describes a wireless LAN system which includes: a plurality of wireless LAN base stations; and a wireless LAN mobile station. Specifically, the wireless LAN mobile station includes: received field strength measuring means for measuring a value of a received field strength; and handover executing means for comparing the value of the received field strength measured by the received field strength measuring means with a threshold value serving as a criterion for executing a handover, to execute the handover when the value of the received field strength is judged to be equal to or smaller than the threshold value. Moreover, the wireless LAN mobile station further includes communication quality measuring means for measuring a communication quality of information that is transmitted and received between the wireless LAN mobile station and the wireless LAN base station associated with the wireless LAN mobile station; and threshold value changing means for increasing threshold value, when the communication quality measured by the communication quality measuring means is judged to be lower than a predetermined communication quality, the threshold value by a predetermined value.
In general, as in the case of the PHS system described in Japanese Unexamined Patent Application Publication (JP-A) No. 2001-112039, the handover is executed according to the preset received electric field strength and the error occurrence status, and hence, interruption (loss) of communications (packets) and a communication-disabled period in which communications are disabled may occur during execution of handover.
As a countermeasure against the interruption (loss) of communications (packets) that may occur during execution of the handover, as in the case of the wireless LAN system described in Japanese Unexamined Patent Application Publication (JP-A) No. 2007-251652, the interruption (loss) of communications (packets) that may occur during execution of the handover is suppressed by measuring the received field strength and the communication quality to properly change the threshold value based on the result of the measurement, and by executing the handover processing with a higher electric field strength and a higher communication quality.
Further, as another countermeasure thereagainst, the interruption (loss) of communications (packets) is suppressed by adopting a high-speed handover processing method to shorten a period necessary for the handover.
However, it is to be noted that handover processing may inevitably become unstable, when high-speed handover processing cannot be performed according to communication environment, or when a repetition of handover processing occurs in order to carry out handover processing under high field intensity and high communication quality.